CONDENSED AND DRIED MILKS

Module 13. Technology of dried milks

Lesson 34
NON FAT DRY MILK

34.1 Introduction

Low-heat nonfat dry milk manufacture requires that heating be carefully controlled to result in minimum amount of induced changes and yet accomplish proper pasteurization. Control of both time and temperature assumes much significance. High temperature short time pasteurization as part of the preheating may be by tubular or plate heaters to not less than 71.6°C at least 15s. Higher temperatures or longer holds contribute directly to whey protein denaturation. This index is used as a measure of the cumulative detrimental heat effects during processing of non fat dry milk. Not more than 10% denaturation should occur.

34.2 Evaporation

Pasteurized skim milk is condensed continuously in an evaporator (or vacuum pan) to 40 to 48% TS. The Baum’e hydrometer is used to test the total solids during condensing. For the conversion of Baum’e reading to total solids, an in-line refractometer may be used for continuous indication of the total solids. For a low-heat product, the evaporator must be designed to operate efficiently with the product temperature below that which causes the denaturation of the proteins. Some types of double and triple effect (or more) evaporators are not suitable for production of low heat product. Therefore, necessary precaution must be taken to be sure that the design and operation of the double effect (or more) evaporator is satisfactory for low-heat product manufacture.

Once the desired total solids are obtained, the condensed skim milk is pumped continuously from the evaporator to a small balance tank. From there the condensed skim milk is pumped through a preheater to raise the temperature to 61.6 to 68.3°C.

34.3 Spray Drying

The high pressure pump, usually of the piston type forces the hot product through the spray nozzle into mist-like droplets in the drying chamber. The pressure of the pump ranges from about 70 to 350 kg/cm² depending on manifold conditions such as nozzle design and size, inlet and outlet air temperature, drying chamber characteristics, particle size, and moisture content desired. Other types of spray nozzles are suitable for low pressure and one utilizes high velocity hot air to achieve atomization of the liquid.

Inlet air is heated by direct flame or steam coils to 120 to 260°C. Drying chamber design and subsequent equipment, as well as climatic conditions and desired moisture in the dry product influence the inlet and exit air temperatures. The exhaust air temperature is the direct guide in controlling moisture of the product. But changes are achieved through adjustment of the inlet air temperature. Low heat nonfat milk is dried to 3.0 to 4.0% moisture. Some driers have an auxiliary heater or re-drier located after the primary drying chamber. These are useful when the air is humid or for enlarging the total drying capacity of the equipment.

34.4 Removal, Cooling and Sifting

Most milk driers have a continuous removal system to immediately separate the dry product from the hot air stream which has been reduced to the range of approximately 80 to 99°C. The dry product should be cooled at once to approximately 32 to 43°C. A few plants still depend upon ambient conditions for cooling. Nonfat dry milk packaged to hot may become lumpy due to "heat-caking" and the development of off-flavor and off-color during storage is much more rapid. A 25 mesh screen with No. 36 wire gage is commonly used for sifting nonfat dry milk ahead of packaging.

34.5 Recommendations to make Low-heat powder

(1) Apply low pasteurization (72°C,15 s),

(2) Begin the evaporating process not above 70°C,

(3) Evaporate the milk not too far,

(4) Keep the concentrate temperature below 60°C,

(5) Cool the concentrate if it is to be kept for a fairly long time,

(6) Maintain the outlet temperature during spray drying at a low level, and

(7) Mix air and droplets in the drying chamber such that no local overheating of the drying droplets can occur.

34.6 High-Heat Non Fat Dry Milk

34.6.1 Pre heating

Except for the total heat treatment before drying, the manufacture of high heat nonfat dry milk is the same as for the low-heat product. In processing high-heat nonfat dry milk, care must be exercised to subject the skim milk to a time-temperature treatment that will impart good bread baking qualities to the nonfat dry milk. During the heat treatment, the whey (serum) protein is denatured. A test for the whey protein nitrogen is used as an index of the suitability of the dry milk for bread. The heat treatment usually is applied to skim milk directly following pasteurization. The general practice is to heat the skim milk in a hot well to 85 to 88°C for the equivalent of 15 to 30 min by a continuous flow of skim milk in the top and later out from the bottom.

34.6.2 Condensing and spray drying

The condensing step parallels the one used for the low heat product, except the evaporator is operated for maximum efficiency without concern about serum protein denaturation. The reheating of the condensed milk ahead of the high pressure pump may be higher than for low-heat nonfat dry milk. Temperature of condensed milk may be elevated to 74 to 79°C or higher. A high temperature improves drying efficiency, but is limited by several conditions. The temperature must be maintained below the level where casein denaturation will cause a high solubility index.

The specific temperature at which the increased solubility index affects the grade of the dry milk is dependent upon

1. Heat stability of the milk,

2. Preheat treatment and

3. Time lapse necessary to allow the condensed milk to flow from the heater outlet through the high Pressure pump to the spray nozzle.

In some plants, the temperature is restricted by the built up of solids on the contact surface of the heater. Since the aim for all plants is to use the condensed milk heater for the entire day's operation (up to 22 hr) before cleaning, the maximum temperature is limited.

34.6.3 Moisture control

The pump pressure ordinarily is within 105 to 350 kg/cm² with the outlet air drying temperature usually between 76 and 99°C. The final moisture in the high heat nonfat dry milk is controlled to approximately 3 to 4 %.

34.6.4 Orifice size

The diameter of the orifice in the spray nozzle is selected on the basis of several factors. Drying chambers with a single spray nozzle have a large orifice. The common range in diameter is 0.272 to 0.449 cm. Orifice size for the multi-nozzle chamber usually is from 0.0635 to 0.132 cm for non fat milk drying. Other design characteristics of the drier affect the orifice size. The inlet or outlet air temperature and pump pressure will influence the orifice size selection and spray pattern. If other conditions are kept constant, a single large orifice increases the average particle size which in turn increases density and sinkability of the particles compared to drying with several nozzles.

The operator must occasionally observe the spray pattern for a malfunctioning nozzle. A particle may lodge in the nozzle and obstruct the spray pattern. An excessively worn nozzle causes an unsatisfactory spraying of the product. Both conditions can cause inadequate drying which may result in milk solids adhering to the drying chamber surface, cause lumps and higher moisture in the dry milk.

34.6.5 Recommendations to make high-heat powder

(1) The milk should be intensely heated, e.g. 90°C, 5 min or 120°C, 1 min; often still higher temperatures are used.

(2) A more intense preheating causes a higher viscosity of the concentrate at the same dry-matter content with all of its consequences.

34.7 Intermediate Heat Non Fat Dry Milk

Intermediate heat nonfat dry milk is from skim milk that has been preheated too much to qualify as low heat, but not enough to meet the high heat requirements. The whey protein nitrogen is more than 1.5 mg per g of nonfat dry milk but less than 6.0 mg. The other processing steps are the same as described for high-heat nonfat dry milk.

34.8 Modified Drying Procedure to Improve Dispersibility

To improve the sinkability or dispersibility of non fat dry milk as an alternative for agglomeration of nonfat dry milk, several processing steps may be modified. Each is changed to affect maximum particle size and uniformity. This involves

(a) Concentrating the skim milk to 45 to 50% TS

(b) A relatively low pressure for spraying

(c) A larger nozzle orifice

(d) The temperature of the inlet drying air is raised to obtain adequate moisture removal from the larger spray droplets

(e) A system of separating the very small particles either by an air stream or sifting is practiced

(f) Seeding the condensed skim milk and partial crystallization of the lactose can also be used.

Under regular conditions of drying, the separation of large particles by screening may yield 10 to 40% of the production. The product with improved dispersibility may command a small premium price. But handling of the remaining product may present a disadvantage. Furthermore, the reconstitutability of nonfat dry milk from the modified drying and screening method is generally inferior to the agglomerated product. It often is darker in color, higher in moisture and has more insoluble material (denatured protein).

34.9 Vitamins Fortification of Spray Dried Non Fat Dry Milk

Nonfat dry milk contains all the vitamins of whole milk except those that are fat soluble A, D, E, and K. Of these, vitamins A and D are most frequently associated with milk, so their restoration in non fat dry milk has merit. Nonfat dry milk may be fortified with vitamin A and D by two procedures:

1. Wet Process: The vitamins in a liquid are diluted with blend edible oil (melting point ~ 37°C) @ 1.5 kg of oil to the vitamins A and D concentrate sufficient for 1000 kg of milk solids. It is important that the oil and vitamins must be thoroughly homogenized to delay oxidation. Ten per cent additional IU of A and D have been recommended to assure minimum requirement after storage. The vitamin mixture may be manually added to vats of fluid skim milk or to the concentrate of known solids content. If the flow of product is reasonably constant or synchronized, the vitamin mixture may be metered into the product. Sufficient mixing must occur for uniform dispersion before drying.

2. Dry Process: A dry form of vitamins A and D may be premixed with non fat dry milk and the premix dry blended into the main lot by batch or continuous procedures. The deterioration of vitamin A in storage is a little slower by the dry blending procedure; however, the vitamin cost and the expense of addition is higher.

34.10 Selection of the Vitamin Concentrates for Fortification

• It should be able to have uniform distribution in the nonfat dry milk.

• The vitamins must not adversely affect the flavor of the nonfat dry milk, either in the fresh condition or after prolonged storage.

• The vitamin potency should remain stable for the shelf life of the fortified product.

• The vitamins also should remain biologically available during storage.

• The vitamin A compounds commonly added are retinyl palmitate and retinyl acetate.

• The vitamin D compounds are rrgocalciferol and activated rehydrocholesterol.

34.11 Manufacture of Non Fat Dry Milk by Drum or Roller Process

The equipment and other facilities for a drum drying operation can be procured for a much lower minimum cost than for spray drying. Consequently, the drum process may be more feasible for a small volume than the spray system. In addition, some industrial users prefer a drum dried non fat and whole milk.

The skim milk should be pasteurized and the heating should be continued to approximately 85°C for 10 min to be sure of good bake test properties. Removal of moisture by condensing to ~ 2:1 will greatly increase the capacity of the drums. If the TS is increased above 18%, the film on the roll becomes increasingly difficult to dry satisfactorily. Higher drum temperature or slower speed will cause scorching of the product despite insufficient drying. The product at 74 to 85°C is pumped into the reservoir between the two drums. Usually the range in moisture content of drum process nonfat dry milk is 3.0 to 4.0 %. Under proper drying conditions, a uniform, thin light (in colour) sheet of dry milk is scraped from each drum. This film drops into a trough. The screw conveyor breaks up the product into small pieces, while being augured to a flaker or hammer mill. Particle size is decreased sufficiently by the mill to permit the passage through the 8-mesh screen during the sifting operation. The product is usually packaged in Kraft bag with a plastic liner reasonably impervious to moisture vapors.

Precautions

1. Scorched particles content (brown and black) may be a problem.

2. In addition to the correct steam pressure, the proper roller conditions are important otherwise, under or over dried milk solids residues contaminate the ensuing product.

Usually quality difficulties are encountered briefly while starting the drying operation.